Bill Gosper <billgosper@gmail.com> wrote:

In all these intervening centuries, has no one thought of anchoring a(n unpowered) skiff with a tiller on a long rope running upstream? With ropes to both shores in case you find it not on your departure side.

I thought that was the standard way of running a river ferry before steam engines were invented: A fixed cable across the river, and a tiller to use the current to push the ferry (which is permanently attached to the cable, but free to move along its length) to one side of the river or the other.